Mixed-metal oxides are produced industrially on a large scale for a variety of uses. Many of these materials are produced in large quantities and in continuous processes such as spray pyrolysis. These processes utilize synthesis methods that involve precursor solutions, typically metal salts, dissolved in a solvent and other dispersing agents. The solution is atomized or delivered by spraying into a reaction chamber, where heat is supplied by either some external means or through subsequent combustion of the solvent. Solid nano-oxide powders result where the mixed-metal oxide powders generally have final stoichiometries determined by the precursor solution compositions. While these processes can operate continuously and generate large output volumes, these dry-precipitate-calcine type methods have limited control over a number of significant material properties, such as crystal structure, homogeneity, morphology, metal dispersion, and other surface properties that can be beneficial to the material performance in a number of applications.
Finer control over mixed-metal oxide synthesis can be obtained using sol gel methods which exploit the tendency of some metal alkoxides or metal organics to form a precursor gel with metal ions intimately mixed (or arranged) in repeating units. The resulting like gel then undergoes decomposition by heating to yield oxides of desired particle size and chemical nature. A closely related method is the Pechini method. In this method, typically an aqueous solution of metal nitrate or other soluble metal salts are mixed with a chelating agent such as citric acid to form a sol. A polyhydroxy alcohol or amine is then added, and the precursor solution is stirred continuously and heated to around 60-70° C. to evaporate substantially all the solvent, and leaving a viscous material comprised of metal chelates bonded to polymer forming species. The dense, viscous gel-like material is then rapidly heated to about 130° C. to initiate polymerization reaction, and the exothermic polymerization reaction raises temperature above 300° C., evolving gases and expanding the polymer network to form a solid metal organic foam material. Generally the material is cooled to about 130° C., stored for about 12 hours, then ramped up from ambient temperature to about 900° C. and held for several hours. During heating, the organic foam decomposes to an amorphous mixed-metal oxide precursor which begins to form crystal structures around 350° C., and is typically completed around 900° C. These methods as stated do generate finer control over crystallinity, size, and other important factors, however as currently practiced they require batch-type processing of a given volume of precursor solution, and further require long time scales in order to conduct solvent evaporations, polymerizations, and oxide generation and crystallization. The batch nature of these processes combined with the relatively long time scales required generally limits the industrial use of these processes severely.
It would be advantageous to provide a methodology whereby a polymeric precursor method could be performed in a rapid, continuous fashion. Such a methodology would allow the synthesis of crystalline mixed-metal oxides in a manner providing finer control over crystallinity, morphology, and characteristics, simultaneously providing a process suitable for industrial use. It would be additionally advantageous if such a process further provided a means by which the relatively long time periods required by current batch methods could be avoided.
Disclosed here is a method for the method for producing mixed-metal oxides using a plurality of solution droplets comprising a polymerizing agent, chelated metal ions, and a solvent. In this methodology, the solution droplets having a diameter less than about 500 μm are discharged into a first region of a reactor for rapid solvent evaporation, transitioned to a second region of the reactor having a generally higher temperature where polymerization is rapidly accelerated generating a metal organic foam material, followed by additional exposure to increased temperature in order to generate a mixture of amorphous and partially crystalline mixed-metal oxide precursors followed by further heating to produce a substantially crystalline mixed-metal oxide. The use of small droplets combined with higher temperatures greatly and dramatically accelerates the rate at which precursor solution may be transformed into crystalline mixed-metal oxides, and additionally generates crystalline oxides with generally enhanced crystallinity, surface area-per-mass ratio, and sphericity over current polymeric precursor methods.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.